1. Field of the Invention
This invention generally pertains to the sensing of fluid flow, and specifically pertains to the sensing of the flow of a gas such as air in the duct work of a heating, ventilating and air conditioning system.
2. Description of the Prior Art
Air flow sensors are used in heating, ventilating, and air conditioning systems to measure and control the distribution of air within primary and secondary ducts that supply air for heating, cooling, or ventilating air conditioned zones within a building.
Several techniques have been used to sense air flow. One technique employs a pitoh tube mounted in the duct work with the pitoh tube connected to a differential pressure sensor. As air flows past the holes in the pitoh tube, a small differential pressure is created. Variations in air flow velocity change the differential pressure in the tube resulting in a change in the output transmitted by the differential pressure sensor. The pitoh tube can be constructed with an array of holes distributed along the tube to sample air velocity at several points within the duct. The distributed array of holes averages the air flow velocity across the duct, offering improved sensing over a single point measurement of air flow velocity. The problem with the pitoh tube and differential pressure sensor technique is that the differential pressures developed with the typical air flow velocity in HVAC systems are very small. Further, the relationship between air flow velocity and differential pressure is non-linear. The small pressures and non-linear relationship both operate to significantly decrease accuracy at low air flow velocities where air flow velocity control is most desired.
A second technique uses variations on a propeller or turbine wheel mounted in or connected in parallel to the air stream with some form of magnetic or optically coupled sensor to detect rotation of the propeller or turbine wheel. As the air moves past the propeller or turbine wheel, it turns on a bearing surface attached to fixed bearing mounts. As the air flow velocity increases or decreases, the propeller or turbine wheel rotates faster or slower respectively, and the connected sensor transmits an output proportional to the air flow velocity. The problem with this technique is that the bearing system and propeller or turbine wheel design limit the accuracy at low air flow velocity. The error becomes increasingly large as air flow velocity decreases toward the stall point, where rotational force on the prop or turbine wheel is less than the frictional forces of the bearing surface. Further, the bearing system will deteriorate in use, causing accuracy to degrade with time until ultimate failure of the moving parts occurs. Contaminants normally present in the air can exacerbate the rate of accuracy degradation in this type of system.
A third technique uses the hot wire anemometer principle. A small sensing element such as a wire is constructed from a material that exhibits the physical property of changing electrical resistance with changes in temperature. The sensing element is placed in the air stream and heated above the temperature of the air. As the air flows by, heat is transferred from the sensing element to the air, cooling the sensing element. Power is delivered to the sensing element to maintain a constant temperature. The amount of heat transferred to the air and the corresponding power delivered to the sensing element are related to the air flow velocity and sensing element surface area. The problem with this technique is that it is sensitive to air flow velocity only in the vicinity of the small sensing element. In heating, ventilating and air conditioning systems, the air flow velocity varies across the cross section of the duct due to duct design or duct installation practice. Therefore, if the sensor is installed in location where air flow velocity is substantially different than the average air flow velocity in the duct, a large measurement error will result. Attempts to address these problems have taken two directions. The first is sensor arrays where the multiple sensing elements are distributed across the duct cross section with their outputs averaged electrically. The second involves sensing elements of larger overall surface area which are distributed across the duct cross section. Both attempts to solve the problem have limitations. The former type of system increases cost and decreases reliability due to the added sensors and averaging circuitry. The latter approach increases the power required substantially, since the entire exposed surface exchanges heat with the air and power increases directly with the increase in the sensing element surface area.
It is clear that there has existed a long and unfilled need for a unit and system for sensing fluid flow velocity which is accurate at low flow velocity, which is insensitive to localized variations in air flow velocity, which is capable of compensating for changes in the ambient temperature of the fluid whose velocity is being sensed, and which requires substantially reduced power with respect to existing systems.